This invention generally relates to decorative laminates and methods of producing the same. More particularly, this invention relates to color registered decorative laminates employing a fully hydrolyzed polyvinyl alcohol modified melamine-formaldehyde resin coating in place of an overlay sheet.
Conventionally, decorative laminates are made of three essential layers: a core layer, a print layer, and a surface layer. The core layer constitutes a bottom or supporting layer onto which the other layers are bonded. In normal high-pressure laminate manufacture the core layer consists of a plurality of cellulosic sheets. The core sheets are generally made from a kraft paper impregnated with a laminating resin. Laminating resins commonly used for the core layer include phenolic, amino, epoxy, polyester, silicone, and diallyl phthalate resins to name but a few. The industrially preferred laminating resin for decorative laminates appears to be a phenolic resin made from the reaction of phenols with formaldehyde. In low-pressure laminate manufacture the core layer is generally comprises of a sheet of particleboard ranging from 3/8" to 1" in thickness.
Placed above the core layer is the print layer which is generally an alpha cellulose pigmented paper containing a print, pattern, or design that has been impregnated with a melamine-formaldehyde resin. Typically, the printing is performed prior to impregnation by a high-speed rotogravure.
The cured melamine-formaldehyde resins are colorless and resistant to light; they are resistant to a variety of solvents and stains; and their heat resistance makes them immune to burning cigarettes, boiling water and heated containers up and stains; and their heat resistance makes them immune to burning cigarettes, boiling water and heated containers up to about 325.degree. F. Without these melamine-formaldehyde resins the decorative laminate industry would not exist as it is known today. However, because these resins are extremely brittle, they sometimes require reinforcement.
The surface layer, or overlay as it is commonly referred to, is a high-quality alpha cellulose paper impregnated with a melamine-formaldehyde resin. This layer protects the print sheet from external abuse such as abrasive wear and tear, harsh chemicals, burns, spills and the like. It is primarily the melamine-formaldehyde resin which accounts for these protective properties. The alpha-cellulose paper acts as a translucent carrier for water-thin resin, imparts strength to the rather brittle melamine-formaldehyde resin, maintains a uniform resin thickness in the overlay by acting as a shim, and controls resin flow.
The core layer, print layer and surface layer are stacked in a superimposed relationship, between stainless steel plates and subjected to a pressure and temperature for a time sufficiently long enough to cure the laminating resins impregnating the respective layers. The elevated temperatures and pressure actually cause the impregnated resins within the sheets to flow which consolidates the whole into an integral mass, known as the laminate. These laminates find use as counter tops, table tops, furniture, store fixtures and the like.
For obvious economic reasons, it is common practice, when producing the paper supported laminates, to consolidate a plurality of these individual laminating assemblies into one large assembly, or press pack, said stacks being separated from one another by a release sheet, and then to laminate this pack by heat and pressure application.
In consolidating the laminate components according to most widely practiced techniques, an individual assembly is placed with its decorative overlayment surface adjacent to a highly polished stainless steel press plate. The function of the press plate is twofold. First, it provides a smooth, defect-free surface to one side of the laminate. Second, in connection with the paper based supported systems, it serves to separate pairs of back-to-back assemblies, thus permitting a plurality of these assemblies to be consolidated into laminates in one operation, usually in back-to-back relationship.
In the earliest days of the high pressure laminating art, the smooth, glossy surface produced during the pressing operation was sometimes, upon customer's request, reduced to a matte finish by rubbing the surface with pumice. Subsequently, a slightly textured surface was produced by pressing the laminate surface against an aluminum foil caul stock. Such a surface was described as mini-textured because the hilltop-to-valley bottom depth of such textures was from about 0.5 mil (0.0005 inches) to about 1.0 mil (0.001 inches). These mini-textured laminates met with immediate success and almost totally replaced the glossy surface market. Somewhat coarser textures or three dimensional surfaces, sometimes called "low-relief" laminates were then produced, e.g. by a printing process known as the "heavy ink" method, described in U.S. Pat. No. 3,373,068. These surfaces had hill-to-valley depths of about 3 to 5 mils.
Finally, very deep, three dimensional textured or embossed laminates were offered commercially. These laminates may be produced by e.g. the methods of U.S. Pat. Nos. 3,860,470, Jaisle et al., and 3,718,496, Willard. Here the hill-to-valley depth in the surface is of the order of about 20 mils.
The techniques used to produce these deep textured surfaces encompass a design latitude heretofore not achieved by producing simulated patterns such as those occurring in natural materials like sandstone, brick, slate, mosaic, marble, leather, rough or weathered timber; even material like rough woven goods such as jute, hemp, etc.
As demands for these new laminates grew, many new designs, such as those simulating tiles or heavy woven cloth, evolved. These designs must have appropriate color contrast and registry with the surface hill-to-valley configuration. Unfortunately, the problem of registration of color and embossing has proven very difficult to solve. The problem has plagued the industry, not only from the standpoint of the appearance of the finished laminate but from the standpoint of the cost of the production of laminates having a substantially perfect registry of color and embossment, since their inception.
One method of overcoming the above deficiencies is set forth in copending applications Ser. Nos. 889,676 and 889,677 filed Mar. 24, 1978, now abandoned and assigned to the same assignee as this invention. According to these applications, there is provided a method of consolidating an assembly comprising a rigidity imparting substrate, a fibrous decor sheet impregnated with a first thermosetting resin, and a second pigmented thermosetting resin layer, which method comprises consolidating the assembly described above under heat and pressure by an embossing press plate having a surface with protuberant and valley areas capable of being impressed into the overlay and fibrous decor sheets, together, an embossment of the decor sheet and a migration of the pigmented resin from the areas of said embossment corresponding to the protuberant areas of the embossed press plates to the valley areas thereof.
Employment of the overlay sheet has, however, generated a host of unwanted problems. The overlay contributes substantial material cost to the manufactured laminate. Not only are there raw material costs involved, such as expensive high-quality alpha cellulose paper and melamine-formaldehyde resins, but there exists substantial processing costs, such as collating sheets, scrap losses generated by the brittle and difficult-to-handle impregnated sheets, as well as the cost of impregnation itself. In addition to these unwanted expenditures and processing steps, the translucent character of the overlay sheet becomes visually disturbing in decorative laminates when, through the use of more advanced printing techniques, a sharper, more intricate design is employed.
Thus, there exists the need for substitution of the overlay sheet by a tough and transparent thermoset layer that will eliminate haze or blur, incur fewer processing steps and lower costs, and yet maintain sufficient resistance to external abuse to be commercially acceptable. The provision for such a layer would fulfill a long-felt need and constitute a significant advance in the art.